How to combine cells inside a cell array of cells to only 2 levels cellarray?

spectra = rand([200, 999999]);
Xcell = mat2cell(spectra, 200, ones(1, 999999));

Xcell(1, [200:295:305045]) = {NaN}; % just adding NaNs in random locations to simulate the reality

[~, edges] = histcounts(1:999999, 129);
edges(end) = 999999;

Xv = cell(1, 128);

    for i = 1:128
      Xv{1, i} = Xcell(1, edges(i)+1:edges(i+1))
    end

Ok. Now I have this Xcell cell array, containing 128 cellarrays, containing a few hundreds cell arrays each, some are with NaNs and some are with spectra. allso important to know that the 128th cell array is smaller than the rest.

At this point i do all sort of things to manipulate my data, and later i want to transform my data into one cell array containing 999999 cell arrays (like in Xcell).

How can i do it?